Cascade semiconductor light sources, in particular, quantum cascade (QC) lasers have been used extensively as mid-infrared sources since their development in the mid-1990's. A detailed account of a preliminary QC laser can be found in U.S. Pat. No. 5,509,025 entitled "Unipolar Semiconductor Laser" issued to F. Capasso et al. on Apr. 16, 1996. In Capasso et al., a QC laser is described that comprises a multilayer semiconductor structure having a multiplicity of essentially identical undoped "active" regions, a given active region being separated from an adjoining one by a doped "energy injection/relaxation" region, which often (for ease of language hereinbelow) is referred to as an "injector" region. In one embodiment, each active region may comprise three coupled quantum wells (QWs) designed to facilitate attainment of population inversion (of course, the use of three QWs is exemplary only, various other structures may be formed to include any desired number of coupled quantum wells). Associated with the coupled wells are at least two (preferably more) energy states with predetermined wavefunctions. In particular, a wavefunction is "associated" with an energy state in a certain well if the centroid of the modulus square of the wavefunction is in this well. The energy injection/relaxation regions are generally selected: (1) to provide substantial energy relaxation and randomization of motion of charge carriers of the given conductivity type in a given graded energy injection/relaxation region when a normal operating voltage is applied; and (2) to inject the charge carriers into the upper laser state of the following active region optical transition.
There are a number of advantages associated with the QC laser structure that have already been recognized by those skilled in the art. Among these are the ability to tailor the emission wavelength, which is discussed below, the high optical power output resulting from the ability to stack (i.e., "cascade") many (stacks from 1-75 have been demonstrated to date) active regions alternated with injection/relaxation regions such that electrons are recycled and emit as many photons as there are active regions in the stack, and the intrinsic reliability of the QC laser structure resulting from the use of well-understood III-V semiconductor materials and the lack of high energy inter-band relaxation mechanisms. Additionally, QC lasers are expected to have a frequency response that is not limited by electron/hole recombination, a narrow emission line because the line-width enhancement factor is (theoretically) zero, and a weaker temperature dependence of the lasing threshold than in conventional (i.e., bipolar) semiconductor lasers. The lasers can have an emission wavelength in the spectral region from the mid-infrared (mid-IR) to the submillimeter region (e.g., 3-100 .mu.m) that is entirely determined by quantum confinement. Advantageously, the emission wavelength within this region can be tailored by controlling the size of the wells and barriers without modifying the composition of the laser structure (the composition being the conventional GaAs- or InP-based material systems). As an alternative, however, the composition may be designed away from a lattice-matched structure in order to achieve higher conduction band discontinuities, a particular interest for short wavelength devices (e.g., .lambda.&lt;5 .mu.m).
Similar to conventional bipolar semiconductor lasers, the QC laser structures developed thus far have generally been limited to emitting at a single wavelength, or a narrow wavelength range around a single center wavelength. There are many applications, such as trace-gas analysis, where multiwavelength sources are desired. In particular, the availability of a dual-wavelength laser would greatly simplify and improve the use of these sources in differential techniques relying on laser pulses of two different wavelengths (one resonant with an absorption line of the target gas and the other off-resonant); one example of such a technique is differential absorption LIDAR (light detection and ranging)--one of the most sensitive spectroscopic methods of pollution monitoring. An article entitled "A multiwavelength semiconductor laser" by A. Tredicucci et al. appearing in Nature, Vol. 396Nov. 26, 1998 at pp. 350-353 discusses a semiconductor laser structure including specific tailoring of the electronic states and electron relaxation times in the superlattice layered structure to achieve several distinct optical transitions (e.g., 6.6., 7.3 and 7.9 .mu.m). A tunable QC laser exhibiting dual-wavelength operation at threshold is discussed in an article entitled "Laser Action by Tuning the Oscillator Strength" J. Faist et al., appearing in Nature, Vol. 387, pp. 777 et seq., 1997. In this case, the two optical transitions originated from separate sections of the same material, biased at different voltages, to obtain two wavelengths via the Stark effect. The wavelengths available with these approaches, however, are limited by the material properties and design of the device or the responsiveness of the device to the applied voltage.